1. Field of the Invention
During the operation of blow molding or stretch-blow molding, certain areas of the preform undergo complex elongations of large amplitude: this is the case of the area which lies immediately below the neck and is which leads to the shoulder of the final container; this is also the case of the bottom area, whatever the shape of the bottom of the final container. It is known, in order to make it easier to stretch the plastic in these areas, locally to soften the material to a greater extent and therefore to heat said preferential areas to a temperature substantially higher than the rest of the preform, while still preventing crystallization of the constituent material of the preform. The problem may furthermore be complicated by the fact that, immediately adjacent to one of these areas to be heated preferentially (for example, the area lying immediately below the neck of the preform and intended to form the shoulder of the final container), may be found an area of the preform which must not be heated or be heated very little in order for it not to be softened and therefore not to undergo deformation during the subsequent steps of blow molding or stretch-blow molding (for example, because the material has already been treated or will be treated subsequently), as is the case, in particular, for the neck of the preform.
Currently, it is known to carry out the heat treatment of the areas which have to be heated preferentially by heating said areas for a longer time and/or more strongly than the rest of the preform. Furthermore, masks are put into place which protect the areas, such as the neck, which must not be heated preferentially.
2. Description of the Related Art
FIG. 1 of the drawings shows highly diagrammatically a conventional arrangement of a tunnel furnace treating the preforms on the run, the linear movement of the preforms being accompanied by a rotation of each preform about itself so that it is exposed approximately uniformly to the radiation from sources which are placed along one side. As shown in FIG. 1, the sources consist of elongate infrared-radiation lamps or tubes 1 placed on top of one another in order to heat the entire body 2 of a preform 3. The effective part of the radiation from each lamp which reaches the preform is denoted by 4, the thermal power received by the preform per unit surface area, for a given emitted power, being greater the shorter the distance from the lamp 1.
The body 2 of the preform 3 has an area A (lying immediately below the neck 5 of the preform) which, having to form the shoulder of the final container and therefore having to undergo significant stretching, must be heated substantially more than the rest of the body. In order to make the heating times for all parts of the body of the preform the same, the distance of each lamp from the area of the preform which has to be heated is regulated and/or adjusted; thus, for example, the area A is heated by reducing the lamp-preform distance; in other words by placing the lamp 1A lying opposite the area A closer to the preform, thereby making it possible to heat this area by decreasing the emitted power. It should be pointed out that this arrangement means that the areas adjacent to the area A themselves run the risk of being heated more extensively if the power is not suitably controlled.
Moreover, in order to attempt to protect the neck 5, which must not be heated, a mask 6 is placed so as to lie substantially level with the junction part between the neck 5 and the body 2 of the preform 3. However, it is found that a fraction 7A (shown by the bold lines) of the radiation 4 emitted by the lamp 1A is not occulted and reaches the neck 5 and that, likewise, a fraction 7B, (also shown by bold lines) of the radiation 4 emitted by the lamp 1B lying above the lamp 1A also reaches the neck 5 (the lamps lying above the lamp 1B are too far away and may be considered as not having an effect on the neck 5). The neck 5 is therefore insufficiently protected and it is thus necessary to limit the power of the lamp 1A, and therefore to limit the additional heating of the area A, in order to avoid heating the neck 5 excessively.
Under these conditions, the current furnace arrangements have the drawback of heating which is insufficiently localized on the areas which have to be heated preferentially, such as the future shoulder, the bottom and possible necking areas. Furthermore, it is difficult, or even impossible, to heat a very narrow area more intensely except by putting into place a local thermal protection system which would excessively complicate the furnace arrangement. With regard to the protection of the areas which must not be heated or must be heated only a little, this can be achieved only by putting into place masks in the form of elongate and profiled plates, the effectiveness of which is not as complete as one would desire.